Technical Field
The present disclosure relates to a fuel cell evaluator for evaluating characteristic of a fuel cell based on electrochemical measurement. Particularly, the present disclosure relates to a fuel cell evaluator using impedance as a value of characteristic evaluation.
Related Art
Electrode reaction in a fuel cell generally progresses through the three stages, i.e. (1) transportation of a reactant to an electrode surface; (2) charge transfer reaction on the electrode surface; and (3) deviation of a product from an electrode surface. To separate and analyze each stage of electrode reaction is very important for evaluation of factors of degradation in characteristic and performance of an electrode, etc. To know the dominant degree of each stage of electrode reaction is very important for evaluation of factors of degradation in characteristic and performance of an electrode, etc.
Particularly, to measure characteristics in a current region not affected by concentration overvoltage is effective for analysis of characteristic of an electrode per se, A tafel slope indicating the relation between the logarithm of current and potential is one of such characteristics. FIG. 15 shows the Tafel slope. As shown in FIG. 15, the Tafel slope varies according to regions b1 and b2. In the region b1, almost of a platinum surface is covered with adsorbed oxygen species (oxide film) so that a Tafel slope caused by reaction called Temkin-Type is observed. On the other hand, in the region b2, all oxide on the platinum surface is reduced so that a Tafel slope caused by reaction on a bare platinum surface called Langmuir-Type is observed.
As a method for separating and indicating overvoltage in a fuel cell, there is a technique disclosed in JP-A-2009-48814. According to this technique, a Tafel slope in a region of very low load (not higher than 0.01 A/cm2) in Tafel plots is obtained and an activation overvoltage is calculated. Then, a film resistance value is obtained from a zero-crossing value on the high frequency side of impedance measurement, and a resistance overvoltage is obtained. Finally, the remaining part of overvoltage is regarded as concentration overvoltage.
Moreover, to measure characteristics in a current region not affected by concentration overvoltage is effective for analyzing characteristic of an electrode per se. A Tafel slope indicating the relation between the logarithm of current and potential is one of such characteristics. There is further known a method in which the horizontal axis is expressed in the logarithm of current density and the vertical axis is expressed in cell voltage so that a fuel cell is evaluated based on the size, etc. of the linear slope (Tafel slope). (see e.g., JP-A-2009-48813)
On the other hand, impedance is a method effective for totally evaluating electrode reaction from frequency characteristic. However, the magnitude of impedance changes sensitively according to change of the current value at a measurement operating point because reaction resistance (Rct) of an electrode is inversely proportional to current (according to the Butler-Volmer equation). For this reason, it is necessary to evaluate impedance while discriminating between reaction stages of the regions b1 and b2.
There may be a case where actually measured impedance may include DC resistance (such as contact resistance, film resistance, etc.) and reaction resistance-independent impedance as in a distributed constant region indicating characteristic of a porous electrode. Because the Tafel slope is characteristic based on reaction resistance as described above, there are circumstances that the relation between the Tafel slope and impedance cannot be obtained accurately before only reaction resistance is extracted when the relation is to be acquired.
It is generally conceived that two Tafel slopes of −60 mV/dec (b1: with adsorbing process) and −120 mV/dec (b2: without adsorbing process) are present in oxygen reduction reaction. here is however a high possibility that the aforementioned region not larger than 0.01 A/cm2 will be a region b1. Accordingly, there is a possibility that activation overvoltage at actually used current density will be undervalued if the Tafel slope (b1) obtained by this technique is used. It is therefore necessary to acquire the second Tafel slope (b2) in order to evaluate activation overvoltage appropriately. There is however a possibility that the influence of concentration overvoltage cannot be ignored in the load region not smaller than 0.01 A/cm2. It is difficult to determine the place affected by concentration overvoltage and it is difficult to obtain the Tafel slope accurately in the region. In addition, because diffusion resistance varies according to the current value at the operating point, it has been heretofore difficult to evaluate diffusion resistance.
In evaluation of characteristic of a fuel cell according to the related art, particularly in evaluation of catalyst, it is necessary to acquire reproducible current-voltage characteristic (Tafel curve) for likelihood evaluation. It is however known that the Tafel curve varies according to the preprocessing condition and the environmental condition of measurement. Therefore, the preprocessing condition and the environmental condition of measurement must be trued up in order to measure the Tafel curve in a stable power generation state of the fuel cell. Accordingly, a lot of labor and man-hour is taken.